Fuel cell flow field having strong, chemically stable metal bipolar plates

ABSTRACT

A bipolar plate ( 30 ) for use in a fuel cell stack ( 10 ) includes one or more first metal layers ( 40   a ) having a tendency to grow an electrically passive layer in the presence of a fuel cell reactant gas and one or more second metal layers ( 40   b ) directly adjacent the one or more first metal layers ( 40   a ). The second metal layer has a tendency to resist growing any oxide layer in the presence of the fuel cell reactant gas to maintain a threshold electrical conductivity. The second metal layer also has a section for contacting an electrode ( 12, 14 ) and providing an electrically conductive path between the electrode ( 12, 14 ) and the first metal layer.

FIELD OF THE DISCLOSURE

This disclosure generally relates to fuel cells and, more particularly,to flow field plates for fuel cells.

DESCRIPTION OF THE RELATED ART

Fuel cells are widely known and used for generating electricity in avariety of applications. Typically, a fuel cell unit includes an anode,a cathode, and an ion-conducting polymer exchange membrane (PEM) betweenthe anode and the cathode for generating electricity in a knownelectrochemical reaction. Several fuel cell units are typically stackedtogether to provide a desired amount of electrical output. Typically, abipolar plate is used to separate adjacent fuel cell units. In many fuelcell stack designs, the bipolar plate also functions to conductelectrons within an internal circuit as part of the electrochemicalreaction to generate the electricity.

Presently, the bipolar plates are made of graphite to provide electricalconductivity. The graphite is also resistant to corrosion within therelatively harsh environment of the fuel cell. However, a significantdrawback of using graphite is that the plate must be relatively thick toachieve a desired strength, thereby reducing power density of the fuelcell stack. Alternatively, there have been proposals to fabricate thebipolar plates out of a metal. However, the metal corrodes in the fuelcell environment, thereby producing an electrically insulating layerthat undesirably increases an electrical contact resistance between thebipolar plate and the cathode and anode electrodes. A relatively thinbipolar plate that resists corrosion is needed to increase the powerdensity and reduce the cost of a fuel cell stack.

SUMMARY OF THE DISCLOSURE

One example bipolar plate for use in a fuel cell stack includes at leastone first metal layer that will grow an electrically passive layer at afirst rate in the presence of a fuel cell reactant gas and at least onesecond metal layer directly adjacent the first metal layer. When indirect contact with the first metal layer, the second metal layer hasthe ability to resist growing a second metal oxide layer in the presenceof the fuel cell reactant gas so that the second metal layer maintains athreshold electrical conductivity suitable for use in a fuel cell. Thesecond metal layer also provides an electrically conductive path betweenthe electrode and the first metal layer.

In one example, a fuel cell assembly includes a cell stack having aplurality of electrodes and a plurality of bipolar plates as describedabove.

An example method of using a bipolar plate as, described above includesresisting growth of any oxide layer on the second metal layer tomaintain a threshold electrical conductivity such that the second metallayer provides an electrically conductive path between the electrode andthe first metal layer.

The various features and advantages of this disclosure will becomeapparent to those skilled in the art from the following detaileddescription. The drawings that accompany the detailed description can bebriefly described as follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates selected portions of an example fuel cell stack.

FIG. 2 illustrates metal layers of a bipolar plate of the example fuelstack cell.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically illustrates selected portions of an example fuelcell stack 10 for generating electricity. In this example, the fuel cellstack 10 includes fuel cells 12 and 14 that each has a cathode 16 thatreceives a first reactant gas and an anode 18 that receives a secondreactant gas to generate an electric current using a known reaction.Each fuel cell 12 and 14 includes a polymer exchange membrane (PEM) 20that separates a cathode catalyst 22 from an anode catalyst 24, and gasdiffusion layers 28 that distribute the reactant gases over therespective cathode catalyst 22 and anode catalyst 24 in a known manner.In one example, the gas diffusion layers 28 include a porous materialsuch as a porous carbon cloth. A metal bipolar plate 30 separates thefuel cells 12 and 14.

The metal bipolar plate 30 includes a first metal layer 40 a coupledwith at least one second metal layer 40 b. In this example, a secondmetal layer 40 b is provided between the first metal layer 40 a and eachof the fuel cells 12 and 14, respectively.

In one example, the second metal layers 40 b comprise a mesh. In someexamples, the second metal layers 40 b are bonded to the first metallayer 40 a using any of a variety of known methods, such as diffusionbonding, brazing, welding, or another method. The first metal layer 40 aprovides the bipolar plate with mechanical strength, and the secondmetal layers 40 b provide the bipolar plate with corrosion resistance tomaintain a desired level of electric conductivity between the fuel cells12 and 14 and the first metal layer 40 a.

In this example, the first metal layer 40 a is solid and continuous andis formed in a known manner to include reactant gas channels 42. Thesecond metal layers 40 b are generally planar mesh sheets that extendover the reactant gas channels 42. The second metal layers 40 b includesections 44 that are in direct contact with the first metal layer 40 aand the gas diffusion layers 28 of the fuel cells 12 and 14 such thatthe sections 44 provide an electrically conductive path between the fuelcells 12 and 14 and the first metal layer 40 a.

FIG. 2 illustrates an example mesh pattern of a second metal layer 40 b.In this example, each second metal layer 40 b includes wires 54 arrangedwith openings 56 in between the wires 54. Given this description, one ofordinary skill in the art will recognize alternative types of meshpatterns suitable to meet their particular needs.

In operation, the electrochemical reactions of the reactant gases withinthe fuel cells 12 and 14 produce a relatively harsh environment for themetal bipolar plate 30. For example, the cathode 16 produces an acidicoxidizing environment and the anode 18 produces an acidic, reducingenvironment. In the disclosed example, the first metal layer 40 a has atendency to grow an oxide layer, such as an oxide scale, in the presenceof the reactant gas. In some examples, the first metal layer 40 a growsoxide scales on surface portions that are not directly coupled to thesecond metal layers 40 b. The second metal layers 40 b also have atendency to grow any oxide layer, but at a slower growth rate relativeto the oxide layer on the first metal layer 40 a. Oxide layers aregenerally poor electrical conductors and tend to reduce the conductivityof the bipolar plate 30. Having the second metal layer(s) 40 b with arelatively slower growth rate facilitates maintaining a selectedelectrical conductivity through the sections 44 of the second metallayers 40 b between the fuel cell electrodes 12, 14 and the first metallayer 40 a even though the growth rate of the oxide layer on the firstmetal layer 40 a might otherwise be too thick to conduct on its own. Insome examples, the second metal layers 40 b may grow an electricallyinsignificant amount of oxide layer. In other examples, there may be nooxide layer at all.

In one example, the threshold for electrical conductivity ispre-selected during a design stage of the fuel cell stack 10 to achievea desired level of performance over a selected useful life of the fuelcell stack 10.

In one example, the first metal layer 40 a is made of a first type ofmetal (or metallic alloy) and the second metal layers 40 b are made of asecond, different type of metal (or metallic alloy). In one example, thefirst type of metal is steel and the second type of metal is a nickel ornickel alloy, titanium or titanium alloy, stainless steel, platinum orplatinum alloy, or a combination of them. The listed metals and metalalloys for the second type of metal provide a desirable corrosionresistance for maintaining a selected electrical conductivity over theelectrically conductive path.

In one example, the second metal layers 40 b are nickel alloy. In oneexample the nickel alloy includes a nominal composition of about 22 wt %chromium, about 14 wt % tungsten, about 2 wt % molybdenum, about 0.5 wt% manganese, about 0.4 wt % silicon, about 0.3 wt % aluminum, about 0.10wt % carbon, about 0.02 wt % lanthanum, up to about 5 wt % cobalt, up toabout 3 wt % iron, up to about 0.015 wt % boron, and the remaining wt %nickel. This example nominal composition provides the benefit of adesirable corrosion resistance for maintaining a selected electricalconductivity over the electrically conductive path. The term “about” asused in this description relative to the compositions refers to possiblevariation in the compositional percentages, such as normally acceptedvariations or tolerances in the art.

In this example, the first metal layer 40 a has a uniform thickness t₁and the second metal layers 40 b have a uniform thickness t₂ that isless than the thickness t₁. The thickness t₁ is suitable for providing adesired amount of mechanical strength to support the bipolar platebetween the fuel cells 12 and 14. The thickness t₂ is suitable formaintaining a desired contact between the first metal layer 40 a suchthat the first metal layer 40 a does not significantly corrode.

The disclosed example metal bipolar plate 30 provides the benefit ofimproved power density compared to previously known graphite bipolarplates. The example metal bipolar plate 30 includes the first metallayer 40 a to provide mechanical strength and the second metal layer 40b to provide corrosion resistance. The corrosion resistance maintains adesired level of electric conductivity to thereby allow the use ofmetallic materials in the relatively harsh environment of a fuel cellstack without significant penalty to conductivity. Moreover, the highstrength of metallic materials compared to graphite allows the examplebipolar plate 30 to be relatively thinner compared to graphite plates.Thinner bipolar plates reduce the cell stack assembly size and providemore power per volume of a fuel cell stack.

Although a combination of features is shown in the illustrated examples,not all of them need to be combined to realize the benefits of variousembodiments of this disclosure. In other words, a system designedaccording to an embodiment of this disclosure will not necessarilyinclude all of the features shown in any one of the Figures or all ofthe portions schematically shown in the Figures. Moreover, selectedfeatures of one example embodiment may be combined with selectedfeatures of other example embodiments.

The preceding description is exemplary rather than limiting in nature.Variations and modifications to the disclosed examples may becomeapparent to those skilled in the art that do not necessarily depart fromthe essence of this disclosure. The scope of legal protection given tothis disclosure can only be determined by studying the following claims.

1. A device for use in a fuel cell stack, comprising: a bipolar platehaving at least one first metal layer having a first oxide layer with afirst growth rate in the presence of a fuel cell reactant gas; and atleast one second metal layer directly adjacent the at least one firstmetal layer, the at least one second metal layer having a second oxidelayer with a second growth rate less than the first growth rate in thepresence of the fuel cell reactant gas such that the second oxide layermaintains a selected electrical conductivity, the at least one secondmetal layer having a section for contacting an electrode and providingan electrically conductive path between the electrode and the at leastone first metal layer.
 2. The bipolar plate as recited in claim 1,wherein the at least one first metal layer is solid and continuous, andthe at least one second metal layer comprises a mesh.
 3. The bipolarplate as recited in claim 2, wherein the at least one first metal layeris contoured to form a reactant gas channel adjacent the section, andthe mesh comprises a planar sheet in direct contact with the section andextending over the reactant gas channel.
 4. The bipolar plate as recitedin claim 1, wherein the at least one second metal layer comprises anickel or nickel alloy, titanium or titanium alloy, stainless steel,platinum or platinum alloy, or combination thereof.
 5. The bipolar plateas recited in claim 1, wherein the at least one second metal layercomprises a nickel alloy.
 6. The bipolar plate as recited in claim 5,wherein the nickel alloy includes a nominal composition of about 22 wt %chromium, about 14 wt % tungsten, about 2 wt % molybdenum, about 0.5 wt% manganese, about 0.4 wt % silicon, about 0.3 wt % aluminum, about 0.10wt % carbon, about 0.02 wt % lanthanum, up to about 5 wt % cobalt, up toabout 3 wt % iron, up to about 0.015 wt % boron, and a remainder wt %nickel.
 7. The bipolar plate as recited in claim 1, wherein the at leastone first metal layer comprises steel.
 8. The bipolar plate as recitedin claim 1, wherein: the at least one first metal layer comprises steel;and the at least one second metal layer includes a nominal compositionof about 22 wt % chromium, about 14 wt % tungsten, about 2 wt %molybdenum, about 0.5 wt % manganese, about 0.4 wt % silicon, about 0.3wt % aluminum, about 0.10 wt % carbon, about 0.02 wt % lanthanum, up toabout 5 wt % cobalt, up to about 3 wt % iron, up to about 0.015 wt %boron, and a remainder wt % nickel.
 9. The bipolar plate as recited inclaim 1, wherein the at least one first metal layer comprises a firstcorrosion resistance corresponding to the first growth rate and the atleast one second metal layer comprises a second corrosion resistancecorresponding to the second growth rate.
 10. A fuel cell assemblycomprising: a plurality of electrodes; and a bipolar plate associatedwith the electrodes, the bipolar plate comprising: at least one firstmetal layer having ability to grow an oxide layer with a first growthrate in the presence of a fuel cell reactant gas; and at least onesecond metal layer directly adjacent the at least one first metal layer,the at least one second metal layer having ability to grow a secondoxide layer with a second growth rate in the presence of the fuel cellreactant gas such that the second oxide layer maintains a selectedelectrical conductivity, the at least one second metal layer having asection contacting one of the electrodes and providing an electricallyconductive path between the one of the electrodes and the at least onefirst metal layer.
 11. The assembly as recited in claim 10, wherein thesection is in direct contact with the one of the electrodes on one sideof the section and in direct contact with the at least one first metallayer on an opposite side.
 12. The assembly as recited in claim 10,wherein the at least one first metal layer is solid and continuous, andthe two second metal layers each comprises a mesh.
 13. A method of usinga bipolar plate that includes at least one first metal layer havingability to grow a first oxide layer in the presence of a fuel cellreactant gas and at least one second metal layer having ability to growa second oxide layer in the presence of a fuel cell reactant gas, themethod comprising: resisting growth of the second oxide layer at the atleast one second metal layer to maintain a selected electricalconductivity in the at least one second metal layer such that the atleast one second metal layer provides an electrically conductive pathbetween a fuel cell electrode and the at least one first metal layer.